1. Field of the Invention
This invention relates to pacers having DC power connecting circuits, and, more particularly, to pacer switching circuits adapted to switchably deliver power from a low power source to a plurality of circuits so as to match the respective circuit requirements to the source characteristics. In the case of low voltage level DC sources, the power connecting circuit may also comprise one or more converter circuits for raising the delivered voltage level.
2. Description of the Prior Art
The cardiac pacer field is now over a decade old, and great strides have been made in improving the characteristics and reliability of pacers for permanent implant in patients. The improvements in the pacer field have come both from improvements in the components which can be used in pacers, such as CMOS devices and other forms of microelectronics, and from improved designs for providing better pacing stimulus production and more reliability. To date, the most significant improvements which have resulted in greater lifetime for an implanted pacer have occurred from circuit improvements which provide for a lower power drain, as well as electrode improvements which permit effective stimulation of the heart with lower output pulses. For the most part during the history of pacer development, there has been relatively little improvement in the battery sources themselves, such that pacer lifetime for most models has been between about 18 months and 36 months. Even with the most advanced hybrid designs which have appeared to date, the current drain causes such a depletion in conventional cells that a lifetime of more than 3 years has not been expected in most models, and is not relied upon by the physician.
However, within recent times new battery sources have become available, and more importantly have been tested to the point where they are being accepted by the industry as reliable. For example, lithium-iodide battery cells are now becoming available from at least several manufacturers, and such cells offer a promise of a lifetime of greater than 5 years, depending of course upon the power drain of the pacer which is being driven. In addition, other types of sources including nuclear sources are becoming available and are gaining acceptance in the industry. Such new power sources give rise to the possibility that reliable and relatively lightweight pacers can be made available, and at an expense not markedly greater than present day models, having lifetimes which are appreciable compared to the statistical expected lifetimes of the patients receiving pacer implants. Studies suggest that the average lifetime of a patient at the time he first receives a pacer implant is approximately five years, and in this light it is recognized that a pacer using a battery source such as a lithium iodide cell which could provide a reliable lifetime of 5 to 10 years would be an outstanding achievement and of inestimable value to a patient receiving such pacer.
While nuclear powered pacers are believed to be feasible having lifetimes in the order of 30 years, such pacers are many times more expensive, and are advisable only for the smaller class of patients where implants are required at a young enough age such that the statistical expected lifetime approaches the lifetime of the pacer source. For the great percentage of the anticipated pacer market, i.e., more than 90%, a pacer which could provide a reliable lifetime of 8 or more years would be considered to be optimum from the standpoint of economy and simplicity. However, when the characteristics of the new battery cells are matched with the electrical requirements of pacers, it is seen that for most pacers the full potential of the newer cells simply is not realizable, primarily because after about 5 years the changing characteristics of the cells produce an altered pacer operation. If such altered operation falls below allowable limits, the pacer has to be removed from the patient even though it may have an appreciable amount of energy left within it.
A first primary characteristic of the lithium-iodide cell is that its maximum voltage output is 2.8 volts. The chemical nature of such a cell provides that in terms of electrical characteristics it presents an ideal 2.8 volt source, in combination with an internal resistance. While the 2.8 volt source remains a fixed constant due to the chemistry of the cell, the internal resistance may and indeed does vary as a function of energy delivered by the battery, i.e., its energy depletion. Roughly speaking, such internal resistance increases linearly with energy depletion until a knee in the curve is reached wherein the internal resistance rises dramatically, at which point the useful life of the source is over very soon. However, if in fact the cell can be utilized throughout the full extent of the linear range of relatively low resistance, and utilized throughout the full extent of this range, then its lifetime can be maximized. It is noted that in this specification the example of the lithium-iodide cell is used for illustrative purposes. For purposes of brevity, the term lithium cell, or lithium battery, is used to denote lithium-iodide, lithium-silver chromate, and other cells of such class. It is to be noted, however, that other types of cells are included as well within the invention, and while they may have differing characteristics, the basic characteristics of the cells are sufficiently similar such that the principles of this invention are applicable. For the lithium-iodide cell, a top voltage of 2.8 volts means that for practical purposes a voltage converter device is required in order to raise the available voltage level to a level which provides a safe voltage for circuit operation. While many modern electronic circuits can be designed to operate at less than 2.8 volts, such a requirement imposes severe design limitations, which limitations drastically increase the required complexity of the pacer circuitry and consequently reduce the resulting reliability. Certain circuits simply require a minimum voltage, such as some CMOS devices which require at least 2.4 volts for operation. Even though such circuits could in fact be utilized, the margin of safety would be virtually negligible, such that a relatively small percentage of energy depletion of the battery source would reduce the voltage to a point where reliable circuit operation would no longer be expected.
In order to more accurately utilize the lithium battery throughout it useful lifetime, it is advantageous to have a means of programming operation so that the pacer characteristics change in a way that can be detected at about the knee in the curve of battery internal resistance. As stated previously, at this point the useful life of the source is over very soon, and an indication of this provides a means for determining that it is time to explant the pacer. Since, in most present day pacer designs, the pulse generator or oscillator which provides the stimulus pulses is inherently a voltage sensitive circuit, such that its frequency varies as a function of the voltage delivered to it, this frequency can be made dependent upon the state of energy depletion of the battery source if the voltage delivered to it is likewise made dependent upon such energy depletion. It is therefore seen that it is advantageous to provide a means for transferring or connecting power from the battery source to the pacer circuitry which interfaces with the battery resistance in a programmed manner, such that the voltage transfer or transmission characteristic is a function of the source internal resistance. By making the programmed transmission characteristic change substantially at the knee, the voltage provided to the stimulus generator is likewise changed substantially when the knee is reached, producing a change of the stimulus signal rate which can be detected.